1. Field of the Invention
This invention relates to improvements in an intake air control system for an internal combustion engine, and more particularly to a control system arranged to prevent degradation in output torque at a high opening degree of a throttle valve while maintaining a high output torque in a low load engine operation.
2. Description of the Prior Art
A variety of intake air control systems for an internal combustion engine have hitherto been proposed and put into practical use. One of them is disclosed in SAE Technical Paper Series No. 880388. FIG. 2 of this SAE Paper Series shows an engine intake air control system in which a rotary slide valve is disposed in an intake air passage upstream of an intake valve. The rotary slide valve is arranged to be opened in timed relation to the opening and closing operations of the intake valve so that air is sucked into a combustion chamber under the descending of a piston during an opening overlap of the rotary slide valve and the intake valve. Here, the pressure prevailing in the intake air passage becomes the level of atmospheric pressure at the initial stage of the opening of the intake valve under the effect of the rotary slide valve, thus reducing pumping loss of an engine.
Additionally, FIG. 9 of the SAE Paper series shows an engine intake air control system in which a throttle valve is diposed in the intake air passage upstream of the rotary slide valve, in which the pressure in the intake air passage is previously lowered below atmospheric pressure by throttling air flow with the throttle valve. As a result, the combustion chamber pressure at the bottom dead center of the piston can be set, for example, at -550 mmHg during idling operation, thereby controlling the intake air amount to be supplied to the combustion chamber. Another example of a similar engine intake air control system is disclosed in Japanese Patent Provisional Publication No. 55-148932 in which a rotary valve is disposed in an intake air passage upstream of an intake valve.
However, difficulties have been encountered in the above-discussed conventional intake air control systems. That is to say, since the rotary slide valve is disposed in series with the intake valve, a complicated gear mechanism made up of a plurality of gears is required to vary the rotational phase of the rotary slide valve. This increases friction loss and therefore totally degrades the reduction effect of pumping loss. Additionally, it is difficult to control the intake air amount for each engine cylinder by such a complicated gear mechanism.